1. Field of Invention
The invention relates to a vibration exciter which is used especially in ground compaction devices.
2. Description of the Related Art
Ground compaction devices, especially vibrating or oscillating plates, comprise a ground contact plate that is movable over the ground to be compacted and is subjected to the action of a vibration generated by a vibration exciter. The shaking movement necessary for this effects a reciprocal sliding and displacement of the substrate particles, whereby intervening spaces can be filled and the desired ground compaction achieved.
Many solutions are known for providing such vibration exciters. In practice, an arrangement that has proven especially successful is one in which two mutually parallel shafts are driven to rotate in opposite directions at the same speed of rotation. Each of the shafts carries one or more centrifugal weights, so that the centrifugal weights also rotate in opposite directions to one another and generate an imbalance. In order to achieve a phase position that is constant over the rotation, the shafts carry mutually engaging gear wheels. The phase position is selected so that the resultant force from the rotating centrifugal weights and hence from the centrifugal forces adopts a desired direction.
It is particularly advantageous if the phase position between the centrifugal weights or eccentric shafts can be adjusted, as a result of which the orientation of the resultant vibration or of the resultant force becomes adjustable to suit the operator""s requirements. For this purpose, it is known, for example from DE 30 43 719 C2, for it to be possible for one of the eccentric shafts to be twisted relative to a gear wheel, which engages with a gear wheel of the other eccentric shaft and generally ensures a solid coupling of the rotational movement.
Other examples of vibration exciters are described in DE 36 09 360 and in DE 35 45 593 C2.
FIG. 5 shows a diagrammatic lateral view of a vibration exciter known from DE 29 09 204 C2.
Two mutually parallel eccentric shafts 1, 2 are driven to rotate in opposite directions and at the same speed of rotation, as indicated by the arrows A and B. Each of the eccentric shafts 1, 2 carries a centrifugal weight 3, 4, as a result of which the centrifugal forces 5 and 6 indicated by arrows arise. For the sake of simplification, the reference numerals will not be repeated in any of the other drawings. The arrangement is the same on each occasion, but in different positions in each case.
FIG. 5a) shows the known vibration exciter, in which the phase position is regulated by a phase regulating device (not shown), thus for example by the gear wheel which is twistable relative to one of the eccentric shafts 1, 2, in a manner such that the centrifugal forces 5, 6 act, in an obliquely upward direction, in other words with a vertical component and a forward-directed horizontal component. For this purpose, a force chart is also shown in FIG. 5a. 
The precise sequence of a 180xc2x0 rotation of the eccentric shafts 1, 2 with the centrifugal forces 5, 6 is shown in 45xc2x0 steps in FIG. 6. The rotation of the centrifugal weights 3, 4 in the constant phase position shown has the effect, because of the resultant forces, that the oscillating plate carrying the vibration exciter is moved forward, toward the left in FIG. 6. The vertical component relieves the load on the ground contact plate or even lifts it slightly above the ground, while the horizontal component provides the desired forward drive.
FIG. 5b) shows the vibration exciter with the phase position changed. In this case the eccentric shaft 2 and the centrifugal weight 4 have been twisted through an angle of 90xc2x0 relative to the eccentric shaft 1 with the centrifugal weight 3 in order to reach the phase position shown in FIG. 5b). In a manner corresponding to this, FIG. 7 shows a 180xc2x0 cycle in 45xc2x0 steps. It is apparent from this that, in the phase position shown in FIG. 5b) and FIG. 7, no resultant horizontal forces arise, while the vertical forces are cumulative to a maximum extent. In this phase position, consequently, the oscillating plate remains stationary and effects a maximum compaction of the ground as a result of maximum vertical movement.
In addition, FIG. 5c) also shows the phase position of the eccentric shafts 1, 2 for the rearward travel of the oscillating plate (toward the right in the figure).
It is common to all vibration exciters known from the prior art that, at what is known as the turn-round or reversal point, in other words in the phase position where the oscillating plate is stationary (FIG. 5b), the resultant centrifugal force vector is oriented vertically. The force acting on the ground to be compacted is thus greatest in this position and reduces during forward or rearward travel of the plate and the swinging of the resultant force vector associated therewith by, for example, 45xc2x0 forward or rearward, to 1/2 of the maximum value.
The arrangements described have proven outstandingly successful in the compaction of earth, sand and gravel, as compaction can be carried out with maximum application of force at selected points.
Problems arise, however, with oscillating plates having vibration exciters such as are to be used for the compaction of asphalt or paving. The problem here is that if the maximum vertical force acts at the reversal point, localized subsidence may occur that can no longer be corrected. With asphalt rollers, therefore, it is customary to switch off the vibration in the reversal area in order to avoid excessive penetration of the roller into the asphalt during the change of direction.
It is an object of the present invention to provide a vibration exciter for ground compaction devices whereby an excessive action on certain substrates, such as asphalt or paving, when the machine is being reversed or is stationary can be avoided.
According to the invention, this object is achieved by a vibration exciter having the features of patent claim 1. Advantageous further developments of the invention can be found in the dependent claims.
A vibration exciter according to the invention for ground compaction devices comprises mutually parallel or coaxial eccentric shafts capable of being driven in opposite directions at the same speed of rotation and each carrying at least one centrifugal weight. In such a vibration exciter, it is a particular feature of the invention that the phase position of the centrifugal weights can be regulated by means of a phase regulating device in a manner such that the vertical components of the centrifugal forces generated by the centrifugal weights in each position of rotation cancel each other out. It is thus made possible, according to the invention, to regulate a phase position in which the vertical components of the centrifugal weights cancel each other out, so that the resultant force of the centrifugal forces comprises no vertical component and no vertical vibration at all acts on the substrate to be compacted. Despite sustained operation of the vibration exciter and thus rotation of the centrifugal forces, further compaction of the substrate can thus be avoided.
A particularly advantageous embodiment of the invention is one wherein the horizontal components of the centrifugal forces act cumulatively in the same direction in this phase position. The resultant force vector thus comprises a maximum horizontal component. This makes it possible to apply a high shearing stress, for example into the asphalt surface, via the ground contact plate in order to smooth over relatively small cracks or pores. As resultant horizontal forces are generated in each of the two directions (forward and rearward) during a 360xc2x0 rotation of the centrifugal weights, the ground compaction device remains virtually stationary.
A further, particularly advantageous embodiment of the invention is one wherein, in order to effect a movement of the ground compaction device in a first direction, the phase position defined for stationary positioning can be adjusted through a positive angle, while in order to effect a movement in a second direction opposite to the first-direction, the phase position can be adjusted through a negative angle. It is thus possible to exploit the advantages already provided by the vibration exciters known from the prior art for the forward and rearward drive of the ground compaction devices.
It is particularly advantageous, however, if, in the event of a change between forward and rearward travel, in other words a change between the first and second directions, the phase position defined according to the invention for the stationary or reversal point can be adopted transitorily. If, then, the operator changes between forward travel and rearward travel, the vertical vibrations are automatically reduced to zero at the reversal point, so that the operator need take no further precautions to protect the substrate to be compacted.
An advantageous further development of the invention is one wherein the phase position can be regulated in a manner such that the horizontal components of the centrifugal forces cancel each other out and the vertical components act cumulatively in the same direction.
As a result, it is also possible to regulate a phase position which ensures the advantages of the vibration exciters previously known from the prior art, in particular a maximum compaction output when the machine is stationary.
A further advantageous embodiment is one wherein the phase regulating device comprises at least one device for superimposing on the contrary rotation of the eccentric shafts the twisting of, in each case, one of the centrifugal weights relative to an associated other centrifugal weight. The phase regulating device, whose structure and mode of operation are known in principle from the prior art printed publications cited above, can thus be combined from a plurality of units in order to satisfy the versatile regulatory possibilities of the phase position provided according to the invention.